This invention relates to the measurement of the duration of voltage pulses in electronic circuits and, more particularly, the duration of a selected pulse in an inverter drive logic pulse train.
Electronic inverters which convert DC voltage to a constant frequency AC output incorporate waveform generating circuits which drive power transistors to produce a quasi sine wave which is filtered to produce a sine wave output. A typical waveform pattern that is used to switch the output power poles of an inverter to produce a quasi sine wave of low harmonic content that can be easily filtered, may include four waveform patterns for each output phase. Two of the waveform patterns would be used to switch a positive semiconductor switch on and off respectively, while the other two waveform patterns would be used to switch a corresponding negative semiconductor switch on and off. Each of the waveforms may include 18 edges. Thus a three phase system may require 216 pulse edges. The relationship between these pulse edges is critical. An error of several microseconds can degrade performance to an unacceptable level. The affected performance parameters are total harmonic content and direct current content. Thus to assure that the circuit is operating properly, a test must be performed on each unit produced to determine that the waveforms have the proper pulse edge relationships.
Various methods have been used to determine if the waveform generating circuit is operating properly. At the printed wiring board level, the waveforms may be examined with an oscilloscope. This is a time consuming task which is prone to error. At the line replaceable unit level, the average DC voltage level may be measured. However, this approach does not produce results of sufficient accuracy since a one percent difference in the average voltage can result in more than doubling of the harmonic content. An alternative approach is to test a complete system in a closed loop configuration by measuring the DC content and harmonic content of the output. This is also a costly and time consuming procedure.
In order to reduce testing time and obtain improved test data, an electronic pulse width measurement circuit was constructed. That circuit responds to a synchronization pulse provided by the inverter drive control circuit and counts pulse edges until the pulse of interest in encountered. A timer is enabled only during the pulse of interest and the output of the timer is subsequently read by a microprocessor. These functions are accomplished by loading a register with the requested pulse number and counting each inverter drive logic pulse up from zero. A four bit magnitude comparator compares the count with the register's pulse number. When the counts are equal, a high speed timer is enabled. On the next inverter drive logic pulse, the timer is disabled. The high speed timer count represents the width of the specified pulse.
The previous circuit is limited by the use of a four bit comparator, such that at most, 15 pulses can be counted after the sync pulse. Furthermore, that circuit requires the use of a separate counter, register, comparator, arming logic and a high speed timer.
It is desired to provide an improved electronic pulse width measurement circuit which requires less printed wiring board space, can be constructed at a lower cost, and permits the counting of a larger number of pulses than the previous circuit design.